Rather than writing your own code to perform these verification steps,we strongly recommend using a Google API client library for your platform,or calling our tokeninfo validation endpoint.

To validate an ID token using the tokeninfo endpoint,make an HTTPS POST or GET request to the endpoint,and pass your ID token in the id_token parameter. For example,to validate the token “XYZ123”,make the following GET request:

CustomJwtHandler.cs

using System;
using System.Collections.Generic;
using System.IdentityModel.Tokens;
using System.Linq;
using System.Net.Http;
using System.Web;
using System.Web.Configuration;
using Newtonsoft.Json;
using System.Net;
using System.Threading.Tasks;
using System.Threading;
using Services.Models;
using System.Security.Claims;

namespace Services
{
    /// <summary>
    ///  This is an implementation of Google JWT verification that
    ///  demonstrates:
    ///    - JWT validation
    /// </summary>
    /// @author kunal.bajpai@gmail.com (Kunal Bajpai)


    public class CustomJwtHandler : DelegatingHandler
    {
        private const string URL_GOOGLE_TOKEN_INFO = "https://www.googleapis.com/oauth2/v3/tokeninfo";

        /// <summary>
        /// 
        /// </summary>
        /// <param name="request"></param>
        /// <param name="cancellationToken"></param>
        /// <returns></returns>
        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken)
        {
            HttpStatusCode statusCode;
            string token;

            var authHeader = request.Headers.Authorization;
            if (authHeader == null)
            {
                // Missing authorization header
                return base.SendAsync(request,cancellationToken);
            }

            if (!TryRetrieveToken(request,out token))
            {
                return Task<HttpResponseMessage>.Factory.StartNew(() => new HttpResponseMessage(HttpStatusCode.Unauthorized));
            }

            try
            {
                ValidateToken(token);
                return base.SendAsync(request,cancellationToken);
            }
            catch (SecurityTokenInvalidAudienceException)
            {
                statusCode = HttpStatusCode.Unauthorized;
            }
            catch (SecurityTokenValidationException)
            {
                statusCode = HttpStatusCode.Unauthorized;
            }
            catch (Exception)
            {
                statusCode = HttpStatusCode.InternalServerError;
            }

            return Task<HttpResponseMessage>.Factory.StartNew(() => new HttpResponseMessage(statusCode));
        }
        /// <summary>
        /// Validates JWT Token
        /// </summary>
        /// <param name="JwtToken"></param>
        private void ValidateToken(string JwtToken)
        {
            try
            {
                using (WebClient wc = new WebClient())
                {
                    TokenInfo tokenInfo = JsonConvert.DeserializeObject<TokenInfo>(wc.DownloadString(URL_GOOGLE_TOKEN_INFO + "?id_token=" + JwtToken));

                    ClaimsPrincipal claimsPrincipal = new ClaimsPrincipal(new ClaimsIdentity(ExtractClaims(tokenInfo),tokenInfo.Issuer));

                    Thread.CurrentPrincipal = claimsPrincipal;
                    HttpContext.Current.User = claimsPrincipal;
                }
            }
            catch (WebException e)
            {
                HttpStatusCode statusCode = ((HttpWebResponse)e.Response).StatusCode;
                if (statusCode == HttpStatusCode.BadRequest)
                {
                    throw new SecurityTokenValidationException();
                }
                else
                {
                    throw new Exception();
                }
            }
        }

        /// <summary>
        /// Tries to retrieve Token
        /// </summary>
        /// <param name="request"></param>
        /// <param name="token"></param>
        /// <returns></returns>
        private static bool TryRetrieveToken(HttpRequestMessage request,out string token)
        {
            token = null;
            IEnumerable<string> authorizationHeaders;

            if (!request.Headers.TryGetValues("Authorization",out authorizationHeaders) ||
            authorizationHeaders.Count() > 1)
            {
                return false;
            }

            var bearerToken = authorizationHeaders.ElementAt(0);
            token = bearerToken.StartsWith("Bearer ") ? bearerToken.Substring(7) : bearerToken;
            return true;
        }

        private List<Claim> ExtractClaims(TokenInfo tokenInfo)
        {
            List<Claim> claims = new List<Claim> {
                new Claim(ClaimTypes.Name,tokenInfo.Name),new Claim(ClaimTypes.Email,tokenInfo.Email),new Claim(ClaimTypes.GivenName,tokenInfo.GivenName),new Claim(ClaimTypes.Surname,tokenInfo.FamilyName),new Claim(ApplicationUser.CLAIM_TYPE_LOCALE,tokenInfo.Locale),new Claim(ClaimTypes.NameIdentifier,tokenInfo.ProviderKey,ClaimValueTypes.String,tokenInfo.Issuer),new Claim(ApplicationUser.CLAIM_TYPE_EMAIL_CONFIRMED,tokenInfo.IsEmailVerifed.ToString(),ClaimValueTypes.Boolean)
            };

            return claims;
        }
    }
}

TokenInfo.cs

using Microsoft.AspNet.Identity.EntityFramework;
using Newtonsoft.Json;

namespace Services.Models
{
    public class TokenInfo
    {
        [JsonProperty("iss")]
        public string Issuer { get; set; }

        [JsonProperty("aud")]
        public string AudienceClientId { get; set; }

        [JsonProperty("sub")]
        public string ProviderKey { get; set; }

        [JsonProperty("email_verified")]
        public bool IsEmailVerifed { get; set; }

        [JsonProperty("azp")]
        public string AndroidClientId { get; set; }

        [JsonProperty("email")]
        public string Email { get; set; }

        [JsonProperty("iat")]
        public long IssuedAt { get; set; }

        [JsonProperty("exp")]
        public long ExpiresAt { get; set; }

        [JsonProperty("name")]
        public string Name { get; set; }

        [JsonProperty("picture")]
        public string Picture { get; set; }

        [JsonProperty("given_name")]
        public string GivenName { get; set; }

        [JsonProperty("family_name")]
        public string FamilyName { get; set; }

        [JsonProperty("locale")]
        public string Locale { get; set; }

        [JsonProperty("alg")]
        public string Algorithm { get; set; }

        [JsonProperty("kid")]
        public string kid { get; set; }

        public override bool Equals(object obj)
        {
            if (obj.GetType() != typeof(ApplicationUser))
            {
                return false;
            }

            ApplicationUser user = (ApplicationUser)obj;
            bool hasLogin = false;

            foreach (IdentityUserLogin login in user.Logins)
            {
                if (login.ProviderKey == ProviderKey)
                {
                    hasLogin = true;
                    break;
                }
            }
            if (!hasLogin) { return false; }

            if (user.FirstName != GivenName) { return false; }
            if (user.LastName != FamilyName) { return false; }
            if (user.Locale != Locale) { return false; }

            return base.Equals(obj);
        }
    }
}

WebApiConfig.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Web.Http;
using Microsoft.Owin.Security.OAuth;
using Newtonsoft.Json.Serialization;

namespace Services
{
    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services
            // Configure Web API to use only bearer token authentication.
            config.SuppressDefaultHostAuthentication();
            config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

            // Web API routes
            config.MapHttpAttributeRoutes();

            config.Routes.MapHttpRoute(
                name: "DefaultApi",routeTemplate: "api/{controller}/{id}",defaults: new { id = RouteParameter.Optional }
            );
            config.MessageHandlers.Add(new CustomJwtHandler());
        }
    }
}